Over the years, the concepts of designs for the total knee arthroplasty have evolved to the point where with few exceptions, most are quite comparable in the design of femoral, tibial and patellar prostheses.
Major discrepancies and problems encountered are caused by physician error and failure to understand the principles of more complex alignment or ligament problems to be corrected at surgery. With the more complex alignment or "routine" degenerative knee, the major differences are the ease and consistency of instrumentation for alignment and proper bone cuts allowing proper ligament balance. This allows satisfactory motion and stability post operatively.
The distal femoral cuts must be placed to provide the knee prosthesis with a proper flexion and extension gap, proper varus-valgus alignment, proper patellofemoral relationship and proper rotation. It is customary to use an intramedullary rod placed in a retrograde fashion between the medial and lateral femoral condyles just anterior to the intercondylar notch to establish a single point of reference for subsequent bone cuts. A major problem is in the instrumentation to indicate the location of the femoral cuts which relies upon the "experience" or "eyeballing" of the surgeon. Over the years, two basic instrument system designs have become popular.
In one design (anterior referencing), the total knee alignment system takes its point of reference from a centrally placed rod and careful attention is given to the patellofemoral joint by using an anteriorly placed feeler gage. The distal femoral cut is consistent with the thickness of the prosthesis.
This instrument system operates on the principle of anatomic anterior and distal femoral cuts to allow proper ligament balancing and stability in extension as well as consistent patellofemoral placement on the anterior surface. The femur is not notched, and the anterior surface of the femoral prosthesis not elevated above the anterior surface of the femur. Notching the femur may cause a decrease in strength of the distal femur. Elevation of the anterior surface of the prosthesis will cause extremely high patellofemoral pressures in a joint that seems to be prone to a high rate of post-operative failure.
By establishing the anterior femoral cut as the benchmark or datum starting point, however, the anterior referencing instruments result in the installation of a knee prosthesis which sacrifices consistent stability in flexion due to the formation of a posterior femoral condylar cut that may leave the posterior space either too wide or too narrow. This can cause instability in flexion, or restrict flexion and cause increased wear.
The second type of instrument design (posterior referencing) is based on the concept that the flexion and extension stability are more important and the patellofemoral joint is of secondary importance. This system also uses an intramedullary rod for referencing. Although I consider all three joints as "important", when a compromise must be made, the posterior referencing systems compromise the patellofemoral joint while the anterior reference systems sacrifice stability in flexion (the posterior tibial femoral joint). Both systems allegedly equally address the distal tibial-femoral space. Neither consistently addresses the distal rotation of the femoral component.
Neither system tries to preserve the joint line at or near an "anatomic" level. By elevating the jointline, the patella is distalized. The femur is also shortened. Since the arthritic knee often has a loss of cartilage, there may be a patella infera of 2-3 mm initially. Elevating the distal femoral resection beyond this will:
1) Further alter the patellofemoral relationship. PA1 2) Change the isometric and rotational balance of the MCL and the LCL. PA1 3) Shorten the femur in flexion and may cause increased roll back, anterior lift off, and increased posteromedial wear. PA1 4) Elevate the level of tibial resection necessitating a major amount of posterior femoral resection to achieve a satisfactory flexion space. PA1 1. Consistent distal tibio-femoral stability. PA1 2. Consistent distal femoral rotation. PA1 3. Consistent placement of the anterior cut flush with the anterior surface of the femoral cortex, i.e., without notching or elevation. PA1 4. Consistent placement of the posterior femoral cut such that the distal and posterior cuts are equal (when indicated) allowing for satisfactory extension and flexion stability and motion. PA1 1. A lax flexion gap; PA1 2. Sloping the proximal tibial cut to accommodate for an inconsistent posterior femoral condylar cut; PA1 3. Significantly notching the femur anteriorly; PA1 4. Raising the anterior flanges of the prosthesis and thus the patellofemoral joint; PA1 5. Not allowing full extension; PA1 6. Raising the joint line; PA1 7. Tightness in flexion; PA1 8. Malrotation; and PA1 9. Patient pain. PA1 determining a prospective planar cut at the posterior of the condyles of the femur at which the distance between the anterior surface of the femoral cortex and the prospective planar cuts is substantially equal to the interior dimension of a knee prosthesis to be fitted on said femur at the anterior surface and the cut planar surface, PA1 determining the thickness of the posterior lateral or medial condyle which will be resected by said prospective planar cut, PA1 cutting the distal ends of the condyles along a plane at which the maximum thickness of resection of the more prominent condyle at said distal end is substantially equal to the thickness determined to be resected at the posterior medial or lateral condyle by said prospective planar cut, and PA1 cutting the condyles along a plane substantially flush with the anterior surface of the femoral cortex, and along said prospective planar cut. PA1 a caliper feeler and measurement plate to measure for the size of the femoral prosthesis to be received, said caliper feeler and measurement plate adapted to determine a first distance between an anterior surface of the femoral cortex and a plane tangent to a posterior surface of the medial and lateral condyles of a femur, the caliper feeler referencing the anterior surface of the femoral cortex and the measurement plate referencing the plane tangent to the posterior surface of the medial and lateral condyles; PA1 a graduated scale to compare the first distance to at least two standard femoral prosthesis sizes and to determine the smaller of the at least two standard femoral prosthesis sizes; PA1 a graduated scale to measure a second distance between the first distance and the size of the smaller standard femoral prosthesis size, so that a thickness or thicknesses can be measured to be resected at the posterior surface of the medial and lateral condyles of the femur by adding the average thickness of the posterior condyles of the smaller standard femoral prosthesis and the second distance; PA1 a tool to resect the medial and lateral condyles along a plane at the anterior surfaces thereof flush with the anterior surface of the femoral cortex; and PA1 a tool to resect distal ends of the medial and lateral condyles at a resected thickness equal to the average thickness of the distal condyles of the smaller standard femoral prosthesis plus the second distance. PA1 measuring for the size of the femoral prosthesis to be received by determining a first distance between an anterior surface of the femoral cortex and a plane tangent to a posterior surface of the medial and lateral condyles of a femur; PA1 using a graduated scale to compare the first distance to at least two standard femoral prosthesis sizes; PA1 measuring a second distance between the first distance and the size of the smaller standard femoral prosthesis size; and PA1 measuring a thickness or thicknesses to be resected at the posterior surface of the medial and lateral condyles of the femur, the thickness being equal to the average thickness of the posterior condyles of the smaller standard femoral prosthesis plus the second distance. PA1 measuring a thickness or thicknesses to be resected at the distal ends of the medial and lateral condyles, the thickness being equal to the average thickness of the distal surface of the smaller standard femoral prosthesis plus the second distance, and resecting the distal ends of the medial and lateral condyles at the measured thickness.
When performing a unicompartmental knee replacement, it is imperative to maintain the jointline. As a consequence, it is desirable to maintain a full range of motion.